1. Field of the Invention
This invention relates to lithographic processes and, in particular, to lithographic processes involving device fabrication.
2. Art Background
Lithographic processes are typically employed in the manufacture of devices such as semiconductor devices. Among the lithographic processes that are available, photolithography is often utilized. Photolithographic processes have the advantage of being suitable for a blanket exposure technique. That is, a material that is sensitive to the exposing light is coated on a substrate, e.g., a silicon wafer, that is being processed to form a plurality of devices. The coating material i.e., the resist, is then subjected to light that has been passed through a mask material so that the light reaching the resist corresponds to a desired pattern that is to be transferred to the underlying substrate. Since the exposure occurs simultaneously over an entire device or a number of devices in process on a wafer, e.g., a silicon wafer, the procedure is considered a blanket exposure.
A blanket exposure procedure is advantageous because it is relatively fast compared to other methods such as the raster scan technique usually employed when the energy used to expose the resist is a beam of electrons. However, generally, resolution obtainable through a blanket exposure with ultraviolet or visible light is somewhat poorer than that achieved with other methods such as electron lithography.
One resist material, poly(methyl methacrylate) (PMMA) upon exposure with ultraviolet actinic radiation has shown resolution better than that achieved with exemplary resists used at conventional wavelengths (greater than 300 nm). For example, PMMA has been shown to be capable of resolution as good as about 2500 Angstroms. (See B. J. Lin, Journal of Vacuum Science and Technology, 12, 1317 (1975).) Although PMMA exhibits excellent resolution, its sensitivity to practical sources of actinic radiation is quite limited. Therefore, exposure times are generally excessive for practical applications. As a result, the attainment of excellent resolution, together with the relatively short exposure times associated with blanket exposure, is an elusive goal.